Method and device for filtering during a change in an ARMA filter

ABSTRACT

A method and device are provided for filtering digital audio signals using at least one ARMA filter, particularly during a filter change. The method includes the following steps: a step of receiving a first request to change filtering to or from filtering by a first ARMA filter; and, in response to the first request, a step of gradually switching, at each of a plurality of cascaded first filtering blocks, between digital-signal filtering by a first basic filtering cell and digital-signal filtering by another associated basic filtering cell, the first basic filtering cells of the plurality of first filtering blocks factorizing the first filter.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is a Section 371 National Stage Application ofInternational Application No. PCT/FR2012/050526, filed Mar. 14, 2012,which is incorporated by reference in its entirety and published as WO2012/123676 on Sep. 20, 2012, not in English.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

None.

THE NAMES OF PARTIES TO A JOINT RESEARCH AGREEMENT

None.

FIELD OF THE DISCLOSURE

The present invention relates to a process for filtering digital signalsby means of at least one autoregressive moving averaged filter,otherwise known as ARMA (Auto-Regressive Moving Averaged) and acorresponding device.

It focuses especially on filtering during modification of the ARMAfilter over time (switching off or activation for example) or during achange of ARMA filter.

A main field of application is that of digital audio signals.

BACKGROUND OF THE DISCLOSURE

As is known per se, a linear ARMA filter can be represented in the formof a transfer function H(z), in a field of z-transforms linking theinput signal X(z) and the filtered output signal Y(z) by:

${Y(z)} = {{{H(z)} \cdot {X(z)}} = {{\frac{b_{1} + {b_{2} \cdot z^{- 1}} + \ldots + {n_{n + 1} \cdot z^{- n}}}{a_{1} + {a_{2} \cdot z^{- 1}} + \ldots + {a_{m + 1} \cdot z^{- m}}} \cdot {X(z)}} = {\frac{B(z)}{A(z)} \cdot {{X(z)}.}}}}$

The polynomial B(z) defines the moving average part (or Moving AverageMA) of the ARMA filter, and the polynomial A(z) defines theautoregressive part (AR) of the ARMA filter.

Finite impulse filters (FIR) correspond in the event where the degree mof the polynomial A is zero.

Infinite impulse filters (IIR) correspond in the event where the degreem of the polynomial A is non-zero.

Because ARMA filters vary over time, and IIR filters in particular, theyare used for example in sound-editing software for dynamically changingeffect (multi-band equalisers, audio effects, etc.), in synthesissolutions (vocal and other) or in processing involving pickup orrestoration of sound (reduction or cancellation of noises, whereof thecharacteristics vary over time).

Modification over time of ARMA filters, and more particularly of IIRfilters, does however have disadvantages, mainly with the appearance oftransitory artefacts during filter change. These artefacts manifest forexample in the form of audio “clicks” particularly unpleasant to hear,and are for example associated with initialisation of internal memoriesof a new IIR filter to be activated, with dephasing introduced by thecurrent ARMA filter to be switched off, and with potential differencesin overall delay (group delay) of the above filters.

Hereinbelow, the description will concentrate mainly on the case offilters of IIR type, linear in particular. In fact, infinite impulseresponse filters are those for which a filter change conventionallygenerates the most artefacts. Also, linear filters prove to be much morewidespread and studied than non-linear filters. In this way, theexplanations hereinbelow could be applied directly in a wide range ofclassic situations.

Nevertheless, it is understood that the invention can apply to any typeof ARMA filter, linear or non-linear, and especially in the case ofVolterra filters which are non-linear polynomial filters. The theory ofso-called polynomial “Volterra” filters is especially documented in thepublication “The Volterra and Wiener theories of nonlinear Systems”(Schetzen, 1989, Ed. Krieger).

Different solutions have already been brought up in techniques of theprior art for trying to limit, or even cancel, the transitory effectslinked to internal memories of these IIR filters.

One approach, illustrated by the publication “Elimination of transientsin adaptive filters with application to speech coding” by Zetterberg, L.H. and Q. Zhang (1988), is based on initialisation of variables ofinternal state in internal memories of the new IIR filter to beactivated.

For correct initialisation, a sufficient number of samples preceding themoment of IIR filter change is stored, then all these samples arefiltered by the new IIR filter. Once initialised, the new IIR filter isapplied to the signal.

The authors of this publication have shown that, contrary to theory, itis possible to be content with a reduced number of samples for executingthis initialisation, despite the infinite character of the impulseresponses of these filters. This number depends directly on the “lengthof the impulse response” of the new filter, a notion defined by the LONGfirst samples accounting for x % of the total energy of the filter (thepercentage x being determined empirically and according toapplications), in the case of a stable IIR filter whereof the impulseresponse tail tends to 0.

This approach however suffers from complexity and/or latency.

In fact, initialisation of internal variables of the new IIR filterrequires filtering of a number of samples N which depends on the LONGlength of the impulse response of the filter: in general, it isnecessary for N>LONG to avoid any audible artefact.

Given this constraint, two options for execution are considered:

-   -   either the internal variables are initialised at the time of IIR        filter change, requiring N+1 samples to be filtered: the N        samples preceding the current sample and the current sample,        though this generates a substantial complexity peak at the        instant t of the IIR filter change;    -   or parallel filtering is carried out with the two filters: the        switching between the two filters is done after N samples. This        option involves latency of N samples before application of the        new IIR filter, meaning that the new filter is applied at the        end of these N samples only. The result is a significant        hindrance if the aim is to apply the new IIR filter to the        signal (for example for cancelling a particular signal) as fast        as possible or again change IIR filter before the end of the N        samples.

These disadvantages are also there in the case of simple deactivation(switching off) of the current IIR filter (in this case, the “newfilter” can be seen as an “identity filter” letting everything passthrough), since the current IIR filter introduces dephasing in thefiltered signal, the origin of audio “clicks” when the filter isswitched off suddenly.

Finally, this approach fails to protect against differences in delay(group delay) between the two IIR filters: if the two filters have adifferent group delay, this inevitably becomes an audible artefact(‘click’ type) linked to discontinuity of the signal.

A hybrid approach has also been covered in the publication “Amodified-superposition speech synthesizer and its applications” byVerhelst, W. and P. Nilens (1986).

This approach uses a “vanishing technique” consisting of instantiatingthe two IIR filters in parallel at the time of switching from one filterto the other, with the following configuration: the new IIR filter,non-initialised, receives the signal to be filtered whereas the formerIIR filter “switches off” (by vanishing) and receives no input.

The filtered output is constituted by the sum of the outputs of each ofthe two IIR filters, a sum formed during a limited period N. Inpractice, the “interpolation” time N (the duration of mixing of theoutputs) necessary for having no audio artefact is greater than the LONGlengths of the impulse responses of the two IIR filters.

This hybrid approach relates to the second execution option mentionedhereinabove for the approach with initialisation of internal memories.In fact, use is also made of two IIR filters operating in parallel.

The difference however is in the instantaneous transition between thetwo IIR filters, made possible due to mixing the two outputs (ratherthan switching instantaneously from an output of one filter to the otherafter a certain time in the second option hereinabove). This limits theartefacts linked to differences in group delay between the IIR filters.

However, this hybrid approach also suffers from other disadvantagesmentioned previously, and especially from latency due to theinterpolation duration which must remain greater than the LONG effectivelength of the impulse response of the IIR filters.

SUMMARY

For this purpose, the invention relates especially to a filteringprocess of a digital signal by means of at least one ARMA filter,comprising a receiving step of a first request for filtering change toor from filtering by a first IIR filter, characterised in that itcomprises, in response to said first request, a progressive switchingstep, at the level of each of a plurality of cascaded first filteringblocks, between digital signal filtering by a first elementary filteringcell originating from factorisation of the first filter and digitalsignal filtering by another associated elementary filtering cell, and

wherein all of said first elementary filtering cells of the plurality offirst filtering blocks factorise said first ARMA filter.

In particular, progressive switching can allow reverse navigatingbetween the first elementary cells and the other elementary filteringcells for example for moving from filtering only by the first ARMAfilter or RM (that is, by the first elementary cells) to filtering bythe other elementary cells (using for example another ARMA filter or IIRor an identity filter representative of the absence of filtering).

The factorisation of an ARMA filter, and especially IIR, extends frombreakdown of the filter to elementary filtering cells whereof all thetransfer functions correspond to factorisation of the transfer functionof said ARMA filter.

Progressive filtering switching extends from one filtering pass toanother, during which the two filtering events are executed together andcombined at least temporarily, that is, for a non-zero instant(corresponding, in a processing technique of a digital signal, to atleast one sample). In other terms, during this progressive switchingphase the two filtering events contribute in part to the output value ofthe filtered signal by a filtering block.

The present invention offers performances improved relative totechniques of the prior art, and especially reduced latency, due inparticular to clearly reduced initialisation time of the internalvariables or phasing time of the filters. In fact, in the first case,the interpolation duration necessary for avoiding audio artefacts itselfis reduced.

This situation results from carrying out, according to the invention,factorisation of the ARMA filter or ARMA filters (or IIR) used in aplurality of elementary filtering cells within filtering blocks, and acontrol, at the level of each of the latter, of switching betweenfilters.

In fact, due to factorisation, the MA and AR filters of the elementaryfiltering cells are of orders less than the corresponding MA and ARfilters of the original ARMA filter (that is, the correspondingpolynomials A and B in the transfer functions are of lesser degrees).The initialisation time of the internal variables of filters of inferiororder is clearly shorter than that of a complete ARMA filter. Thereduced initialisation time is obtained by progressive filteringswitching at the level of each of the elementary filtering cells (ofinferior order) during activation of the corresponding ARMA filter.

The same applies during simple switching off of the current ARMA filter.

The present invention enables increased reactivity, for example when anew filter must be instantiated rapidly in response to an unforeseenacoustic event. This is especially the case when the aim is to cancel asignal which appears suddenly (a pure sound, a multifrequency signalwith two tonalities or DTMF, etc.) and/or whereof the temporal/spectralcharacteristics vary rapidly over time.

It shall be noted that dynamic switching from or to an ARMA filter canthus be done instantaneously by applying a switching operation to theoutput of each elementary filtering cell which then serves as input ofthe following elementary cell (due to the cascading of said firstfiltering blocks).

Also, due to faster initialisation, any peak in complexity at theinstant the switching linked to the simultaneous instantiation of twofilters is clearly reduced.

During progress from the state wherein the digital signal is filteredbefore switching (therefore either only by the first elementaryfiltering cells factorising the first ARMA filter, or by the otherelementary cells representing another filter, “identity” or not) to thestate wherein the digital signal is filtered after switching (thereforeby the other elementary cells or by the first elementary cells), thefiltered output signal of each filtering block combines a signalfiltered by the first elementary cell and a signal filtered by the otherassociated elementary cell. For example, progressive switching at thelevel of a first filtering block comprises the combination, at leasttemporary, of a filtered signal by the first elementary filtering cellwith a filtered signal by the other associated elementary filtering cellto produce a mixed output signal of filtering block during progressiveswitching.

In other words, the signals filtered by a first elementary cell and bythe elementary cells with which is associated are mixed, that is,combined within the same filtered signal. Mixing guarantees smoothing ofthe transition between the filtering states, and consequentlycontributes to attenuation, or even cancelling, of audible artefactsduring such transition.

Therefore, a filtering block can employ an adder (or mixer) which addscontributions resulting from the filtering outputs of the two elementarycells of the block. As will become evident hereinbelow, the switchingcontrol means can be varied, for example by varying the contributions ofeach elementary cell by application of a progressive switchingcoefficient to their output signal, or by acting directly on the inputsignal of these elementary cells or by application of a similarswitching coefficient (for example in the case of linear cells), or byuse of an interrupter shunting the signal to be filtered to one or theother of the elementary cells of the block. In the latter caseespecially, the elementary filtering cell which is disconnected by theinterrupter continues to supply an output signal due to the time ofimpulse response, ensuring a kind of mixing of output signals of the twoelementary cells of the block at the level of the adder. Progressiveswitching is therefore carried out.

In an embodiment, said progressive switching can use fades whereof thecorresponding fade coefficients are applied to the signals filtered bythe first elementary filtering cells and by the other elementaryfiltering cells, typically reverse fades, one closing and the otheropening, within the same first filtering block.

The opening fade and the closing fade are also known by the Englishterms “fade-in” and “fade-out”, designating progressive increase andprogressive decrease in the amplitude of a signal. So, applying fade atthe same time to all the first elementary filtering cells progressivelyswitches off the first ARMA filter (closing fade of these elementarycells) or it is activated progressively (opening fade), retaining theapproach by elementary cell enabling fast initialisation of internalvariables.

Similarly, filtering resulting from the cascading of associatedelementary filtering cells in the filtering blocks (corresponding toanother ARMA filter or an “identity” filter for example) is alsoactivated or switched off progressively, in reverse order to the firstARMA filter.

By way of variant, switching can be immediate, for example byapplication of the above “vanishing technique”. But progressiveswitching, “fade-in/fade-out” type, limits the appearance of audibleartefacts, and switches more rapidly from one ARMA filter or IIR to theother, with “constant” artefact.

According to a particular characteristic, fades applied to the signalsfiltered by the first elementary filtering cell and by the otherelementary filtering cell of the same first filtering block are reversefades, one opening and the other closing, the corresponding fadecoefficients of which vary over time.

Fade coefficients represent a mixing coefficient between the twosignals, representative of mixing according to the fade-in/fade-outfunction. This arrangement shows that the ARMA filter change combines aset of switching events at the level of each filtering block. It will beensured preferably to conduct simultaneous switching, and optionallywith the same mixing coefficient, for all the filtering blocks. However,other uses are feasible within the scope of the present invention, suchas application of different fade coefficients (in form and/or duration)from one filtering block to the other, or even in a given filteringblock, between said first elementary cell and the associated elementarycell.

According to a configuration of the invention, within each firstfiltering block, said other elementary filtering cell is placed inparallel with said first elementary filtering cell to which a switchingcoefficient is applied, and said other elementary filtering cell uses anidentity filter weighted by a complementary switching coefficient. Thisconfiguration simply uses a control mechanism of the simple activationor simple switching off of said first ARMA filter. In fact in this case,in the absence of activation of the latter (therefore either prior toswitching or after the latter), only cell identity filters are cascaded,ensuring that an overall filter identity is acquired, that is, nomodifying filtering.

In this configuration, the first elementary filtering cell and the cell“identity” filter in parallel receive the same digital signal at input(that of output of a preceding filtering block). They also supply acommon output signal combined by means of a mixing coefficient (fadecoefficients such as mentioned earlier also) whereof the componentassigned to the cell “identity” filter (the switching coefficient) isreverse or complementary to the (fade) coefficient assigned to the firstassociated elementary filtering cell.

This complementarity can be linked to the above mixing coefficient whichspecifies the degree of progressiveness in switching. It also relates tothe fact that the contributions of the first elementary filtering celland of the cell “identity” preferably must not introduce uselessattenuation or amplification of the digital filtered signal. In thiscase, the sum of the two coefficients applied to the first elementarycell and to the parallel cell “identity” filter could be equal to 100%(amplitude average), or the sum of their squares could be equal to 100%(energy average). Of course, other scenarios of complementarity betweenthe applied coefficients are also feasible.

In a particular embodiment, two broken down ARMA filters, each offiltering blocks of elementary cells described previously, can be placedin series to switch from one to the other. In this case, said pluralityof first filtering blocks is placed in series of a plurality of secondcascaded filtering blocks, each comprising a second elementary filteringcell originating from factorisation of a second ARMA filter placed inparallel with a cell identity filter and weighted by a switchingcoefficient, all the second elementary filtering cells of the pluralityof second filtering blocks factorising the second ARMA filter, and

at the level of the second filtering blocks the process comprisesprogressive switching of reverse filtering by the second elementarycells of the progressive filtering switching by the first elementarycells at the level of the first filtering blocks to control progressiveswitching between filtering by the first ARMA filter and filtering bythe second ARMA filter.

Therefore, switching between two ARMA filters is managed in modularfashion by placing corresponding filtering blocks in series decomposingthe preferred ARMA filters, and by navigating them in reverse.

According to another configuration of the invention, the two IIR filtersbroken down in elementary cells can be instantiated in parallel. In thiscase, within said first filtering blocks, said other elementaryfiltering cell is placed in parallel with said first elementaryfiltering cell and comprises a second elementary filtering celloriginating from factorisation of a second ARMA filter.

When placed in parallel, the first and second elementary filtering cellsof a filtering block receive and filter the same digital input signal ofthe filtering block (corresponding to the output signal of the upstreamfiltering block), and their filtered signals can be combined givenswitching from one to the other (that is, in practice as a function of amixing coefficient, for example) to generate the filtered output signalof the relevant filtering block.

In particular, if one of the factorisations of the first and second ARMAfilters comprises more elementary filtering cells than the other, a cellfilter of identity filter type is associated with each supernumeraryelementary filtering cell to form said filtering block. This arrangementeffectively deals with the case of ARMA filters not having the sameorder (for the MA or AR part, for example).

Since switching is progressive, it is not rare for a new request forchange of ARMA filter to arrive while switching is incomplete, that is,while the two filtering events within a filtering block each contributein part to the output signal of the latter.

Therefore, according to an embodiment of the invention, in response to asecond request for reverse filtering change of the first request (thatis, returning to filtering as it was before the start of said switching)and received during progressive switching, it can be provided to reversesaid filtering switching from the switching state corresponding to theinstant of reception of said second request for change. This inversioncan be realised by reversing applied fade coefficients, or by reversalbetween increase and decrease of applied fade coefficients, from theirrespective values at the time of receipt of the new request.

This arrangement keeps the internal memories of the filters unchangedand prevents discontinuities in the audio signal (generating unpleasantartefacts), while processing of the new request is not delayed. It isparticularly useful for some audio applications wherein the same filteris regularly deactivated and reactivated (for example, for low-volumeacoustic shocks where the activation decision can be unstable).

In another embodiment of the invention, in response to a third requestfor change of ARMA filter to a new ARMA filter received during saidprogressive switching, the process also comprises a progressiveswitching step, at the level of each first filtering block, betweendigital signal filtering by said first filtering block in the statecorresponding to the instant of reception of said third request anddigital signal filtering by a new elementary filtering cell originatingfrom factorisation of said new ARMA filter.

The progressive combination of fade-in/fade-out type can be done betweenoutput of an elementary cell of the third ARMA filter and output of afiltering block in the state it was in during reception of the newrequest for change of ARMA filter. It is thus easy to modularly andrecursively integrate a large number of ARMA filters while they areactivated while the preceding transition (preceding switching) is notcomplete.

The resulting filtering is smoothed despite the large number of ARMAfilter changes.

According to an embodiment of the invention, the first and secondelementary filtering cells are filters of the first or second order. Theorder of a filter, and therefore of the elementary filtering cells, isespecially the greatest of degrees of the polynomial numerator anddenominator constituting their transfer function. As a result,initialisations of the internal variables to each elementary cell arelast much less than conventional techniques for a complete ARMA filter.As a consequence, this arrangement ensures brief switching time betweenthe two ARMA filters.

In particular, the first and second elementary filtering cells arefilters of the first or second order with real coefficients. Thisarrangement has factorisation operations, that is, breakdown operationsof the ARMA filter in elementary filtering cells, which employ onlyslightly complex calculations.

According to a particular characteristic, the first and secondelementary filtering cells are filters of the first order resulting frombreakdown in lattice of the first and second ARMA filters. Using filtersof the first order ensures minimal initialisation time of the internalvariables and therefore of switching between ARMA filters.

As a variant, the first and second elementary filtering cells arefilters of the second order. This arrangement offers less complexity(fewer elementary cells to be instantiated) and retains very shortinitialisation times.

Correlatively, the invention relates to a device for filtering a digitalsignal by means of at least one ARMA filter, comprising:

-   -   a plurality of cascaded first filtering blocks, each comprising        a first elementary filtering cell and another associated        elementary filtering cell, all of said first elementary        filtering cells of the plurality of first filtering blocks        factorising a first ARMA filter;    -   at least one control element for controlling, at the level of        each first filtering block, a filtering change in response to a        first request for filtering change to or from filtering by the        first ARMA filter, and    -   each so-called first filtering block is configured to operate        the filtering change by progressive switching between digital        signal filtering by the first elementary filtering cell and        digital signal filtering by said other associated elementary        filtering cell.

Software, material or hybrid implementation of filters digital is knownper se. The device according to the invention can be used for software,equipment or hybrid.

The device according to the invention has advantages similar to those ofthe process explained hereinabove, especially for reducing latencyduring change of ARMA filter.

Optionally, the device can comprise means relating to thecharacteristics of the process mentioned earlier.

In an embodiment, each so-called first filtering block comprises anadder for at least temporarily combining a filtered signal by the firstelementary filtering cell with a signal filtered by said otherassociated elementary filtering cell to generate a mixed filtering blockoutput signal during progressive switching. In another embodiment, theat least one control element comprises progressive attenuation means ofthe filtered signal by the first elementary filtering cell and of thefiltered signal by said other associated elementary filtering cell priorto combination, according to respectively two complementary fadecoefficients, one closing fade and the other opening fade.

According to a configuration of the invention, within each firstfiltering block, said other elementary filtering cell is placed inparallel with said first elementary filtering cell to which a switchingcoefficient is applied and said other elementary filtering cell uses anidentity filter weighted by a complementary switching coefficient.

In particular, in “series” usage, said plurality of first filteringblocks is placed in series of a plurality of cascaded second filteringblocks, each comprising a second elementary filtering cell originatingfrom factorisation of a second ARMA filter placed in parallel with acell identity filter and weighted by a switching coefficient, all ofsaid second elementary filtering cells of the plurality of secondfiltering blocks factorising the second ARMA filter, and

at the level of the second filtering blocks the device comprises controlelements of progressive filtering switching by the reverse secondelementary cells of the progressive filtering switching by the firstelementary cells at the level of the first filtering blocks to controlprogressive switching between filtering by the first ARMA filter andfiltering by the second ARMA filter.

According to a “parallel” configuration, within said first filteringblocks said other elementary filtering cell is placed in parallel withsaid first elementary filtering cell and comprises a second elementaryfiltering cell originating from factorisation of a second ARMA filter.

Especially, if one of the factorisations of the first and second ARMAfilters comprises more elementary filtering cells than the other, a cellfilter of identity filter type is associated with each supernumeraryelementary filtering cell to form a so-called filtering block.

The invention also relates to means of storing information comprisinginstructions for a computer adapted to execute the filtering processaccording to the invention when this program is loaded and run by acomputer system.

The invention also relates to a computer program readable by amicroprocessor, comprising instructions for executing the filteringprocess according to the invention when this program is loaded and runby the microprocessor.

The means for storing information and computer program havecharacteristics and advantages similar to the processes which theyconduct.

BRIEF DESCRIPTION OF THE DRAWINGS

Other particular features and advantages will emerge from the followingdescription, illustrated by the attached diagrams, in which:

FIG. 1 schematically illustrates a first embodiment of the invention;

FIGS. 2, 2 a and 2 b illustrate operating examples of fade out carriedout in the progressive change of IIR filters according to the invention;

FIG. 3 schematically illustrates a second embodiment of the invention;

FIG. 4 schematically illustrates another embodiment of the filteringdevice according to the invention; and

FIG. 5 shows a particular material configuration of a device or systemcapable of executing the process according to the invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Examples for the present illustration are two digital linear filterswith infinite impulse response or “IIR filters” or “recursive filters”,designated h₁ and h₂, both having the same even order, designated L, fortheir respective parts MA and AR.

The invention naturally also applies to other cases of ARMA and IIRfilters.

For example, it applies to non-linear filters (especially Volterrafilters), to FIR filters, and also in the event where both IIR filtersh₁ and h₂ have non-even and/or different orders, or in the event wherefor the same IIR filter h₁ or h₂ the parts MA and AR also have non-evenand/or different orders. The expert will be able to adjust the followingexplanations to these different cases by obtaining for example a cell ofthe first order as a complement to cells of the second order, or byobtaining elementary filtering cells whereof the denominator or thenumerator is a constant while the associated numerator or denominator isa polynomial of a degree at least equal to 1. In particular, the abovepublication “The Volterra and Wiener theories of nonlinear Systems”provides indications on obtaining factorisation of a non-linear systemof elementary cells.

As is known per se, linear IIR filters h_(i), (i=1 or 2) are defined bythe z-transform

${{H_{i}(z)} = {\frac{B_{i}(z)}{A_{i}(z)} = {G \cdot \frac{\prod\limits_{l = 0}^{\frac{L}{2} - 1}{\left( {1 - {\beta_{i,l} \cdot z^{- 1}}} \right)\left( {1 - {\overset{\_}{\beta_{i,l}} \cdot z^{- 1}}} \right)}}{\prod\limits_{l = 0}^{\frac{L}{2} - 1}{\left( {1 - {\alpha_{i,l} \cdot z^{- 1}}} \right)\left( {1 - {\overset{\_}{\alpha_{i,l}} \cdot z^{- 1}}} \right)}}}}},$

where H_(i)(z) is the z-transform of the filter h_(i), G is the gain ofthe filter H_(i), and α_(i,l) and β_(i,l) are complex coefficients.

The transfer function H_(i)(z) can also be factorised in cells of thesecond order as follows:

$\begin{matrix}{{H_{i}(z)} = \frac{B_{i}(z)}{A_{i}(z)}} \\{= \frac{\prod\limits_{l = 0}^{\frac{L}{2} - 1}\left( {b_{i,l,0} + {b_{i,l,1} \cdot z^{- 1}} + {b_{i,l,2} \cdot z^{- 2}}} \right)}{\prod\limits_{l = 0}^{\frac{L}{2} - 1}\left( {1 - {a_{i,l,1} \cdot z^{- 1}} - {a_{i,l,2} \cdot z^{- 2}}} \right)}} \\{{= {\prod\limits_{l = 0}^{\frac{L}{2} - 1}{h_{i}^{l}(z)}}},}\end{matrix}$

where a_(i,l) and b_(i,l) are real coefficients.

These formulae show how an IIR filter h_(i) can be factorised in cellsof the first order and/or in cells h_(i) ^(l) (l=0 . . . L/2−1) of thesecond order, that is, its transfer function is broken down into aproduct of polynomial cells of the second degree. The first IIR filterh₁ is factorised in a plurality of a first elementary filtering cells h₁^(l) and the second IIR filter h₂ is factorised in a plurality of secondelementary filtering cells h^(l) ₂.

It is evident here that the above generalisation on any IIR filter ofany dimensions is immediate, given that any filter can be factorised asa ratio of products of cells of the first and of the second order withreal coefficients (lattice breakdown, into cells of the second order).

According to the invention, any other breakdown with cells of greaterorders (than the second order) can also be used, if necessary. However,breakdown into cells of the real first and of the second order will bepreferred, since the less complex corresponding calculations contributeto better optimisation of the invention.

Note x(n) the digital audio signal to be filtered (X(z) its z-transform)and y(n) the resulting filtered signal (Y(z) its transform), n being thesample index.

The aim of the present invention is managing the filtering during an IIRfilter change: for example, activation of an IIR filter in the previousabsence of filtering; switching off an IIR filter for the sake ofabsence of filtering; shifting from one IIR filter to a new IIR filter(as described hereinbelow) or to a modified version of the same currentfilter.

Such a change can be requested by an operator via a user interface, orby another system (for example noise reduction software). In thesevarious instances, a command for IIR filter change is received.

Reference is made to the example of deactivation of an IIR filter h₂,current for activation of a second filter. In other terms, this is thefilter change h₁ in h₂ at time T:

${y(n)} = \left\{ {\begin{matrix}{{{x(n)}*h_{1}},} & {\forall{n < T}} \\{{{x(n)}*h_{2}},} & {\forall{n \geq T}}\end{matrix}.} \right.$

The other examples derive smoothly from this example: for example,taking “identity” filters (H(z)=z) for h₁ or h₂ to respectivelyillustrate simple activation of h₂ or simple switching off of h₁.

FIG. 1 schematically illustrates a first embodiment of the invention formanaging this switching at the time T from the filter h₁ to the filterh₂. The elementary filtering cells shown in this figure, and theassociated control elements can easily be implemented via softwareand/or material elements.

This first example corresponds to a filtering device 1 implementing “inseries” the filters h₁ and h₂. As emerges from the figure, each filterh_(i) constitutes a “block” which is easy to handle to be connected inseries to another “block” representing another IIR filter. Therefore, itis easy to “connect” in series or substitute any filter to anotherfilter, for the purpose of executing the present invention.

In this configuration, each filter h_(i) is constituted by the cascadingof elementary filtering h_(i) ^(l) cells which break it down. l is anindex of elementary filtering cells depending on completed breakdown.For example, for breakdown into elementary cells of the first orderonly, L cells are obtained and the index l varies from 0 to L−1.Similarly, for breakdown into elementary cells of the second order only,L2 cells are obtained and the index l varies from 0 to L/2−1.

Associated with each elementary filtering cell h¹ ₁ at output is acontrol element 10 _(i) ^(l), for example a potentiometer module forapplying an attenuation factor or “fade coefficient” to the signalfiltered by h₁ ^(l). The whole of these two elements is called cellfilter CF^(l) _(i). It should be noted that in software usage, thiscontrol element can consist of a fade coefficient applied directly tothe signal leaving the corresponding elementary filtering cell.

As emerges from the following explanations, a cell filter CF^(l) _(i)comprises an elementary filtering cell and an associated control elementfor filtering an input signal. According to different configurations,the control element can be placed after the elementary cell or beforethe elementary cell.

In the case of placement after the elementary cell (in FIGS. 1 and 3 forexample), this control element cannot be simple instantaneous switchingof interrupter (or switch) type but offers progressive transition (fadetype, especially).

In the case of placement before the elementary cell (as in FIG. 4, forexample), this control element can prove to be a simple interrupter. Infact, the impulse response time of the elementary cells (these cellshave a “memory”) ensures non-zero contribution of these “disconnected”cells at the level of corresponding adders, and therefore progressiveswitching according to the invention at the level of each filteringblock. With a control element of progressive type (fade coefficient forexample), this contribution due to “memory” is also modified by the fadecoefficient applied by the control element.

In the example of FIG. 1, the control element uses a fade in/fade outfunction during switching between the two filters h₁ and h₂.

The control elements 10 ^(l) ₁ controlling the first elementaryfiltering cells h₁ ^(l) are preferably identical, simultaneouslyapplying the same closing fade coefficient C_(f) during switching to theIIR filter h₂ (for example progressively from 100% to 0%).

The control elements 10 ^(l) ₂ controlling the second elementaryfiltering cells h^(l) ₂ are preferably identical, simultaneouslyapplying the same opening fade coefficient C_(o) during the sameswitching (for example progressively from 0% to 100%). In particular,the opening fade coefficient C_(o) can be complementary to the closingfade coefficient C_(f), that is, for example C_(o)+C_(f)=100%. It shouldbe noted that the ratio C_(o)/C_(f) is designated a mixing coefficient.

Also, each cell filter CF^(l) _(i) (that is, an elementary filteringcell and its associated control element 10 ^(l) _(i) is placed inparallel with a cell “identity” filter CF^(l) _(PTi) composed by anelementary filtering cell of “identity” PTi type (that is, lettingeverything pass through, represented symbolically by a dotted line inthe figure) and an associated control element 10 ^(l) _(PTi) of the sametype as the control elements 10 ^(l) _(i). This placing in parallelconsists of applying the same input signal to the two elementary cellsand acquiring an output signal adding up the respective output signalsof each cell filter CF^(l) _(i) and CF^(l) _(PTi).

The two cell filters CF^(l) _(i), CF^(l) _(PTi), therefore receive thesame input signal {tilde over (s)}_(i) ^(l=1)(n) and respectivelygenerate a filtered output signal to an adder ADD (or mixer). The latteradds the two acquired filtered signals to generate a filtered outputblock signal. During progressive switching, this output block signalmixes the two filtered signals, the intensity of the contribution ofeach elementary filtering cell h^(l) _(i), PTi being a function of thefade coefficient applied by the associated control element 10 ^(l) _(i),10 ^(l) _(PTi).

The “filtering block” B^(l) _(i) is called the unit constituted by thetwo filter cells CF^(l) _(i), CF^(l) and by the adder ADD. A filteringblock therefore receives an input signal to be filtered and generates afiltered output signal which is the input signal filtered by one of thetwo elementary filtering cells H^(l) _(i) or PTi in a period ofstabilised operation (permanent state) or which is, during progressiveswitching (transitory state), a combination (mixing) of the input signalfiltered respectively by each of the two elementary filtering cells.According to the positioning of the control element in the filter cells,the degree of contribution of the filtering from each of the elementarycells is a function of the applied switching coefficients and/or of the“memory” (impulse response) of these elementary cells.

In detail, the control element 10 ^(l) _(PTi) is complementary to thecontrol element 10 ^(l) _(i): the control elements 10 ^(l) _(PT1)implement the opening fade coefficient C_(o), while the control elements10 ^(l) _(PT2) implement the closing fade coefficient C_(f). In otherterms, each branch PT1 behaves relative to the fade coefficient as acell filter CF^(l) ₂ of the second IIR filter (vice and versa for PT2and the filter cells CF^(l) ₁.

In the case of breakdown of the IIR filters into elementary cells of thesecond order, there is s_(i) ^(l)(n) with

$\left( {{i \in \left\{ {1;2} \right\}},{l \in \left\{ {0,\ldots\mspace{14mu},{\frac{L}{2} - 1}} \right\}}} \right)$the output (filtered signal) of each elementary filtering cell h^(l) ₁^(l) and s_(i) ^(l)(n) the output of the filtering block B^(l) _(i) thatis, the result of output mixing of the signal passing through the cellfilter CF^(l) _(i) with the signal passing through the cell filterCF^(l) _(PTi), (“identity”) in parallel.

These outputs are shown as follows for the filter h₁:

$\mspace{79mu}{{s_{1}^{l}(n)} = {{\sum\limits_{k = 0}^{2}{b_{1,l,k} \cdot {{\overset{\sim}{s}}_{1}^{l - 1}\left( {n - k} \right)}}} + {\sum\limits_{k = 1}^{2}{a_{1,l,k} \cdot {s_{1}^{l}\left( {n - k} \right)}}}}}$${{\overset{\sim}{s}}_{1}^{l}(n)} = \left\{ \begin{matrix}{{s_{1}^{l}(n)} = {{{\overset{\sim}{s}}_{1}^{l - 1}(n)}*h_{1}^{l}}} & {n < T} \\{{{C_{f}\left( {n - T} \right)} \cdot s_{1}^{l}} + {\left( {1 - {C_{f}\left( {n - T} \right)}} \right) \cdot {{\overset{\sim}{s}}_{1}^{l - 1}(n)}}} & {T \leq n < {T + N}} \\{{\overset{\sim}{s}}_{1}^{l - 1}(n)} & {n \geq {T + N}}\end{matrix} \right.$

with the convention {tilde over (s)}₁ ⁻¹(n) (input signal) and where thefade coefficient C_(f)(n) is a function of fade out generally decreasingand monotone (examples shown in FIG. 2) and C_(o)=1−C_(f) in thisexample.

N designates the switching duration between the two IIR filters. It canbe determined empirically or by experimentation and is selected lessthan the initialisation durations of the techniques of the prior art. Ncan be selected as equal to a LONG value such as defined previously butcalculated for one of said elementary filtering cells, for example thebiggest length covering 80% of the energy of an impulse response(infinite) among all the elementary filtering cells.

Correspondingly, the outputs relating to the filter h₂ are shown asfollows, for C_(o)=1−C_(f):

${{s_{2}^{l}(n)} = {{\sum\limits_{k = 0}^{2}{b_{2,l,k} \cdot {{\overset{\sim}{s}}_{2}^{l - 1}\left( {n - k} \right)}}} + {\sum\limits_{k = 1}^{2}{a_{2,l,k} \cdot {s_{2}^{l}\left( {n - k} \right)}}}}}$${{\overset{\sim}{s}}_{2}^{l}(n)} = \left\{ \begin{matrix}{{\overset{\sim}{s}}_{2}^{l - 1}(n)} & {n < T} \\{{\left( {1 - {C_{f}\left( {n - T} \right)}} \right) \cdot s_{2}^{l}} + {{C_{f}\left( {n - T} \right)} \cdot {{\overset{\sim}{s}}_{2}^{l - 1}(n)}}} & {T \leq n < {T + N}} \\{{{\overset{\sim}{s}}_{2}^{l}(n)} = {{{\overset{\sim}{s}}_{2}^{l - 1}(n)}*h_{2}^{l}}} & {n \geq {T + N}}\end{matrix} \right.$

with the input signal convention

${{\overset{\sim}{s}}_{2}^{- 1}(n)} = {{{\overset{\sim}{s}}_{1}^{\frac{L}{2} - 1}(n)}.}$

It should be noted that if in this example the same fade coefficientC_(f) (via C_(o)=1−C_(f)) is applied to h₂ as for the filter h₁, thelatter can be replaced by a different fade coefficient C_(f)′corresponding to a function similar to C_(f) but the temporal supportand the form whereof can be different.

Also, the invention also applies if the second IIR filter h₂ is placedin front of the first filter h₁ (contrary to the figure).

In steady state before the instant T, the input signal x(n) is filteredonly by the IIR filter h₁:y(n)={tilde over (s)}_(s) ^(L/2−1)(n)={tilde over (s)}₁^(L/2−1)(n)={tilde over (s)}₁ ^(L/2−2)(n)*h ₁ ^(L/2−1)={tilde over (s)}₁⁻¹(n)*h ₁ ⁹ * . . . * h ₁ ^(L/2−1) =x(n)*h ₁.

This corresponds to a configuration of the device 1 wherein thecoefficient C_(f)=100% and the coefficient C_(o)=0%. In fact, the signalx(n) is successively filtered by each of the first elementary filteringcells h^(l) _(i) then passes through the second part of the device viathe “identity” branches PT2. The contributions of the branches PT1 andh^(l) ₂ are not considered due to the coefficient C_(o)=0%.

At the instant T, switching between the two IIR filters h₁ and h₂ beginsfor a period N.

In the present embodiment of the invention, the principle is toprogressively switch off the first IIR filter h₁ by progressivelyreducing the coefficient C_(f) (and by increasing C_(o) complementarily)so that the input signal passes progressively through the “identity”branches PT1. This progressive reduction is conducted via fade out.

Similarly, the second IIR filter h₂ is progressively lit by increasingC_(o) complementarily to C_(f) (this is for example fade in),progressively diminishing the signal passing through the branches PT2.

In other terms, during the N switching samples the digital signal isfiltered by the two IIR filters h₁ and h₂, their respectivecontributions varying according to C_(f).

For each pair (H^(l) ₁, H^(l) ₂), ∀l, switching at the level of theelementary cells consists of progressively decreasing the contributionof the elementary filtering cell coming from the first IIR filter h₁ forthe sake of contribution of the other elementary cell of the pair(coming from the IIR filter h₂).

In each filtering block B^(l) _(i) this switching consists ofprogressively decreasing the contribution of the first elementaryfiltering cells h^(l) ₁ (respectively cell “identity” filters CF^(l)_(PT2)) to the benefit of the contribution of associated cell “identity”filters CF^(l) _(PT1) (respectively the second elementary filteringcells h^(l) ₂).

According to the invention, this switching control is undertaken at thelevel of each filtering block (and of each elementary filtering cell) bymeans of appropriate means controlling the coefficients C_(o) and C_(f)applied by the control elements 10 ^(l) _(i) and 10 ^(l) _(PTi).Therefore during these N samples, as a filtered output signal for eachfiltering block B^(l) _(i), for example the barycentre of the outputsignal of the preceding block (via the connection PTi) is calculatedwith the result of this signal filtered by the elementary filtering cellh^(l) _(i) respectively affected by a fade coefficient (C_(f) for theelementary cells of h₁, and C_(o)=f(C_(f)) for example C_(o)=1−C_(f) forthe elementary cells of h₂) and of the complementary coefficient. Thesecoefficients favour signals filtered by h₁, at the start of switching(C_(f) close to 1), and favour signals filtered by h₂ on completion ofswitching (C_(f) close to 0).

On completion of switching (n≧T+N), the digital signal x(n) is filteredby the IIR filter h₂ only. In fact, C_(f)=0 and therefore:

$\begin{matrix}{{y(n)} = {{\overset{\sim}{s}}_{2}^{{L/2} - 1}(n)}} \\{= {s_{2}^{{L/2} - 1}(n)}} \\{= {{{\overset{\sim}{s}}_{2}^{{L/2} - 2}(n)}*h_{2}^{{L/2} - 1}}} \\{= {{{\overset{\sim}{s}}_{2}^{- 1}(n)}*h_{2}^{0}*\ldots*h_{2}^{{L/2} - 1}}} \\{= {{{\overset{\sim}{s}}_{1}^{{L/2} - 1}(n)}*h_{2}}} \\{= {{{\overset{\sim}{s}}_{1}^{- 1}(n)}*h_{2}}} \\{= {{x(n)}*h_{2}}}\end{matrix}$

The first IIR filter h₁ is no longer being used. It can be deactivatedsuch that only the second IIR filter h₂ is retained to continueprocessing. Fresh switching of the latter to another filter can be doneby applying the ideas of the invention again with the breakdown of thisother filter in series (in this case the coefficient C_(o) applied tothe elementary cells h^(l) ₂ becomes a closing fade coefficient C_(f)).

It is not rare, during switching from one IIR filter to another, for anew request for IIR filter change to arrive while switching isincomplete. The new request arrives for example at the time T₁ withT<T₁<T+N.

This can be a request for switching back to the first IIR filter h₁.

In this case, it can be provided that in response the filteringswitching is reversed from the switching state at the time when this newrequest is received.

For example, if at this instant T₁ the coefficients C_(f)(T₁) areapplied to the filter h₁ and C_(o)(T₁) to the filter h₂, reversingswitching consists of applying a new closing fade coefficient C_(f) tothe filter h₂ and a new opening fade coefficient C_(o) to the filter h₁,substantially taking the form of FIG. 2 (over a period N′ which can beequal or different to N), but with values of equal origin, respectively,at C_(o)(T₁) and C_(f)(T₁) (FIG. 2a ).

Reprising these values at origin ensures the absence of discontinuity inthe filtered signal, and therefore of audible artefacts.

In a variant reducing return time to the filtering state by h₁ theprofile of the applied fade coefficients can be retraced in the reversedirection. In the case for example of a symmetrical profile, this cancorrespond to being placed instantaneously at the time T′₁ and toapplying the end of the coefficient profile of FIG. 2 and reversingC_(f) (applied to 10 ^(l) ₁ and 10 ^(l) _(PT2)) and C_(o) (applied to 10^(l) ₁ and 10 ^(l) _(PT2)), with C_(f)(T′₁)=C_(o)(T₁) andC_(o)(T′₁)=C_(f)(T₁) to avoid any discontinuity (FIG. 2b ). Inparticular with C_(o)=1−C_(f), T′₁=N′=T₁−T), and the transition durationfor returning to filtering by h₁ is N′=T₁−T.

It can also be a request for selecting a new (third) IIR filter h₃.Breakdown of the latter similar to those for the filters h₁ and h₂ iscarried out to acquire third elementary filtering cells

${h_{3}^{l}\text{:}\mspace{14mu}{H_{3}(z)}} = {\prod\limits_{l = 0}^{\frac{L}{2} - 1}{{h_{3}^{l}(z)}.}}$

In this case, it can be provided in response to the new request thatfresh filtering switching is carried out from the switching state at thetime when this new request is received. For this, the mixing state ofthe instant T₁ is fixed (that is, the coefficients C_(f)(T₁) andC_(o)(T₁) and all of the first and second IIR filters h₁ and h₂ areconsidered as a single filter (designated h₁₂) having this fixed state,in series of which the new filter h₃ is placed (for example withinfiltering blocks placed in series of the filtering blocks comprisingh₁₂).

The preceding ideas are applied to the filters h₁₂ (starting filter) andh₃ (end filter), especially the presence of associated cell “identity”filters in parallel of each elementary cell of h₁₂ and h₃, as well asnew closing fade coefficients C_(f)′ (for h₁₂) and opening C_(o)′ (equalto 1−C_(f)′, for h₃).

It should be noted that to simplify execution, the coefficient C_(o)′ isapplied to each elementary filtering cell h^(l) ₃ (via control elements10 ₃ ^(l)) and the coefficient C_(f)′ at fixed values C_(f)(T₁) andC_(o)(T₁) is applied to the control elements 10 ^(l) _(i), alreadyexisting for filters h₁ and h₂.

FIG. 3 schematically illustrates a second embodiment of the inventionfor managing this switching at the time T of the filter h₁ to the filterh₂. The elementary filtering cells shown in this figure, as well as theassociated control elements, can easily be utilised via software and/ormaterial elements.

This second example corresponds to a filtering device 1 wherein thefilters h₁ and h₂ are directly instantiated “in parallel”. Eachelementary filtering cell h^(l) ₁ of the first IIR filter h₁ isassociated with an elementary filtering cell h₂ of the second IIR filterh₂ within filtering blocks B^(l). In this way using the elementary“identity” cells PTi is avoided.

Each pair (h^(l) ₁, h^(l) ₂) accompanied by the respective controlelements has the same properties as the pair of filtering blocks (B^(l)₁, B^(l) ₂) of FIG. 1. Accordingly, switching between the two IIRfilters is done in the same way.

In this configuration, L/2 pairs corresponding to L/2 filtering blocksB^(l) are cascaded.

The output {tilde over (s)}^(l) (n) of a filtering block B^(l) is shownas follows, for C_(o)=1=C_(f):

${{\overset{\sim}{s}}^{l}(n)} = \left\{ {{\begin{matrix}{s_{1}^{l}(n)} & {n < T} \\{{{C_{f}\left( {n - T} \right)} \cdot s_{1}^{l}} + {\left( {1 - {C_{f}\left( {n - T} \right)}} \right) \cdot {s_{2}^{l}(n)}}} & {T \leq n < {T + N}} \\{s_{1}^{l}(n)} & {n \geq {T + N}}\end{matrix}{with}{s_{i}^{l}(n)}} = {{\sum\limits_{k = 0}^{2}{b_{i,l,k} \cdot {{\overset{\sim}{s}}^{l - 1}\left( {n - k} \right)}}} + {\sum\limits_{k = 1}^{2}{a_{i,l,k} \cdot {s^{1}\left( {n - k} \right)}}}}} \right.$at the level of each elementary filtering cell h_(i) ^(l) and theconvention {tilde over (s)}⁻¹(n)=x(n).

The ideas provided previously in connection with the first example arealso applicable to this second example of a filtering device 1. This isthe case in particular for managing a new request for IIR filter change,for the form of functions C_(f) and C_(o), or again the order of theelementary filtering cells.

It should be noted that in the event where the two IIR filters h₁ and h₂have parts AR and/or MA of different orders, the number of elementaryfiltering cells can be different to each other. In this case, one ormore elementary cells of “identity” type (PTi) is created which isassociated in parallel to each supernumerary elementary filtering cellof the IIR filter having a larger number of elementary cells h^(l) ₁ toform a filtering block B^(l). Therefore, each elementary cell of the twoIIR filters forms part of a filtering block.

FIG. 4 schematically illustrates a degraded example of execution of thefiltering device according to the invention.

In this example, the control elements 10 ^(l) _(i) and 10 ^(l) _(PTi)are positioned within the filter cells before the elementary filteringcells h^(l) _(i) and PTi. FIG. 4 corresponds to the particular casewhere these control elements employ instantaneous transition(C_(f)(n<T)=1; C_(f)(n≧T)=0), represented here by a single interrupter10 ^(l).

In this case progressiveness of the switching results from the impulseresponse duration of the elementary filtering cells, according to whichthe latter continue to generate an output signal (switching off signal)even in the absence of input signal. In fact, the adder ADD combinesthis switching off signal with the signal filtered by the otherelementary “active” cell, ensuring progressive switching.

It is however noted that the duration of progressive switching is lesswell controlled here than in the case of FIGS. 1 and 3, where theprofile of the coefficient C_(f) directly controls the switchingduration N.

In reference again to FIG. 4, the control elements 10^(l) _(i) aresingle switches or interrupters which do not apply progressiveswitching. But they produce initialisation durations of the internalvariables which are considerably reduced due to use of elementaryfiltering cells h^(l) _(i) according to the invention at the level ofwhich switching is controlled (command 0/1 shown in the figure forcontrolling simultaneously all the interrupters 10 ^(l)).

This degraded embodiment can perform just as well in the “series”version of FIG. 1 as in the “parallel” version of FIG. 3.

The invention such as described hereinabove therefore reduces latency inan IIR filter change even more significantly since these IIR filters arebroken down into elementary filtering cells having low orders.

Some tests have shown notable efficacy of the invention with respect toknown techniques. For example, applying the invention to controlswitching between the IIR filters of order 4, factorised into twoelementary cells of order 2, the inventors reduced by half the switchingtime relative to the hybrid method mentioned previously, with constantartefact energy (audio click).

The present invention has various applications in the field of digitalsignal processing, and especially audio signals, since the use of IIRfilters varying over time is very widespread.

By way of illustration, this is the case of linear prediction coders(LPC-speech coder). In this case, the filter change LPC from one frameto the other is generally done by interpolation of coefficients. Usingthe present invention improves filtering stability during switching byhaving minimal latency and complexity.

This is also the case for acoustic shock suppressor modules. Anytelephone terminal plans to integrate limiters on the loudspeaker signalthe task of which is to protect the user from potential acoustic shocks(strong signals, Larsen signals, etc.). For example, these limiters arefound in self-contained units for the use of representatives, in VoIPclients, in mobile terminals.

In the case of Larsen, the frequential content generally varies rapidlyover time. If the frequencies which compose the Larsen over time aresupposedly known, using the invention quasi instantaneously adapts theIIR filters to these frequencies and filters the Larsen effect.

Finally, it is also the case of noise reduction systems. Such systemsare used in any terminal fitted with a sound pickup, especiallyhands-free types.

In this case, using noise reduction filters would employ filters whichvary over time to adapt to the spectral content of word and/or noise.Applying the ideas of the invention to these systems especially improvesthe latter with respect to latency.

FIG. 5 schematically shows a device or system 50 for executing theinvention, especially equipment fitted with IIR filters and on which thefilter switching is carried out.

The system 50 comprises a communications bus 51 to which are attached acentral processing unit or “microprocessor” 52, live memory 53,non-volatile memory 54, a display device 55 for displaying userinterfaces, a pointing device 56 and optionally other peripherals 57(communications interface, diskette or disc reader, etc.).

The non-volatile memory 54 comprises the programs, execution of whichexecutes the process according to the invention, and for examplesoftware definitions of IIR filters, optionally already broken down intoelementary filtering cells.

During execution of programs, the executable code of these programs isloaded in live memory 53, RAM type, and executed by the microprocessor52. This execution allows instantiation of the IIR filters and controlof their activation and their switching off, that is, also of theswitching from one IIR filter to the other, as shown in FIGS. 1, 3 and 4for example.

The display device 55, such as a screen, enables display of graphic userinterfaces for example allowing a user to generate activation, switchingoff or switching IIR filter commands.

The pointing device 56 can be integrated into the display device,especially when it is a touch screen, or remote, for example a mouse, atouch pad or a graphic tablet, to allow the user to send these commands.

The device described here and the central unit 52 in particular arelikely to employ all or part of the processing described in connectionwith FIGS. 1 to 4 for executing the processes of the present inventionand constituting the devices and systems of the present invention.

The preceding examples are only embodiments of the invention which isnot limited thereto.

For example, the invention can employ only the “block” h₁ of FIG. 1(left part), corresponding to the case of FIG. 3 wherein the filter h₂is an identity filter (the second elementary filtering cells are also“identity” cells). Switching according to the invention switches off thefilter h₁ for the sake of absence of filtering, without artefact orreducing it significantly.

Similarly, using only the “block” h₂ of FIG. 1 (right part), switchingaccording to the invention activates the filter h₂ whereas initially nofiltering of the digital signal was performed, without artefact orreducing it significantly.

An embodiment of the present invention eliminates at least one of thedisadvantages discussed in the Background section.

The invention claimed is:
 1. A process comprising: receiving a digitalsignal by a filtering device comprising a plurality of first cascadedfiltering blocks, each filtering block comprising a first elementaryfiltering cell and another associated elementary filtering cell, all ofsaid first elementary filtering cells of the plurality of firstfiltering blocks being configured to factorise a first autoregressivemoving average (ARMA) filter; filtering the digital signal by thefiltering device to produce a filtered digital signal, wherein filteringcomprises: receiving by a processor of the filtering device a firstrequest for a filtering change to or from filtering by the first ARMAfilter by the filtering device, controlling at least one control elementto progressively switch the filtering device between digital signalfiltering by the first elementary filtering cell and digital signalfiltering by said other associated elementary filtering cell at a levelof each of a plurality of first cascaded filtering blocks in response toreceiving the first request.
 2. The process as claimed in claim 1,wherein the progressive switch at the level of a first filtering blockcomprises the combination of a filtered signal by the first elementaryfiltering cell with a filtered signal by the other associated elementaryfiltering cell to produce a mixed output signal of a filtering blockduring the progressive switch.
 3. The process as claimed in claim 2,wherein said progressive switch uses fades whereof fade coefficientscorresponding are applied to the signals filtered by the firstelementary filtering cells and by the other elementary filtering cells.4. The process as claimed in claim 2, wherein within each firstfiltering block, said other elementary filtering cell is placed inparallel with said first elementary filtering cell to which a switchingcoefficient is applied, and said other elementary filtering cell uses anidentity filter weighted by a complementary switching coefficient. 5.The process as claimed in claim 4, wherein said plurality of firstfiltering blocks is placed in series with a plurality of cascaded secondfiltering blocks, each comprising a second elementary filtering celloriginating from factorisation of a second ARMA filter placed inparallel with a cell identity filter and weighted by a switchingcoefficient, all of said second elementary filtering cells of theplurality of second filtering blocks factorising the second ARMA filter,and at the level of the second filtering blocks the process comprisesprogressive switching of filtering by the second reverse elementarycells of the progressive filtering switching by the first elementarycells at the level of the first filtering blocks to control progressiveswitching between filtering by the first ARMA filter and filtering bythe second ARMA filter.
 6. The process as claimed in claim 2, whereinwithin said first filtering blocks said other elementary filtering cellis placed in parallel with said first elementary filtering cell andcomprises a second elementary filtering cell originating fromfactorisation of a second ARMA filter.
 7. The process as claimed inclaim 6, wherein if one of the factorisations of the first and secondARMA filters comprises more elementary filtering cells than the other, acell filter of identity filter type is associated with eachsupernumerary elementary filtering cell to form said filtering block. 8.The process as claimed in claim 1, comprising, in response to a secondrequest for filtering reverse change of the first request and receivedduring the progressive switch, an inversion step of said filteringswitching from the switching state corresponding to the instant ofreception of said second request for change.
 9. A filtering device for adigital signal, comprising: a plurality of first cascaded filteringblocks, which receive the digital signal and produce a filtered digitalsignal, each filtering block comprising a first elementary filteringcell and another associated elementary filtering cell, all of said firstelementary filtering cells of the plurality of first filtering blocksbeing configured to factorise a first autoregressive moving average(ARMA) filter; at least one control element configured to control, atthe level of each first filtering block, a filtering change to or fromfiltering by the first ARMA filter; a processor, which is configured to:receive a request for a filtering change to or from filtering by thefirst ARMA filter; control the at least one control element toprogressively switch the filtering device between digital signalfiltering by the first elementary filtering cell and digital signalfiltering by said other associated elementary filtering cell at a levelof each of the plurality of first cascaded filtering blocks, in responseto receiving the request.
 10. The device as claimed in claim 9, whereineach said first filtering block comprises an adder for combining asignal filtered by the first elementary filtering cell with a signalfiltered by said other associated elementary filtering cell to generatea mixed output signal of a filtering block during progressive switching.11. The device as claimed in claim 10, wherein the at least one controlelement comprises progressive attenuation means of the filtered signalby the first elementary filtering cell and of the filtered signal bysaid associated elementary filtering cell before combination, accordingto two complementary fade coefficients respectively, one closing fadeand the other opening fade.
 12. The device as claimed in claim 9,wherein within each first filtering block said other elementaryfiltering cell is placed in parallel with said first elementaryfiltering cell to which a switching coefficient is applied and saidother elementary filtering cell uses an identity filter weighted by acomplementary switching coefficient.
 13. The device as claimed in claim12, wherein said plurality of first filtering blocks is placed in seriesof a plurality of cascaded second filtering blocks, each comprising asecond elementary filtering cell originating from factorisation of asecond ARMA filter placed in parallel with a cell identity filter andweighted by a switching coefficient, all of said second elementaryfiltering cells of the plurality of second filtering blocks factorisingthe second ARMA filter, and at the level of the second filtering blocks,the device comprises control elements of progressive filtering switchingby the second reverse elementary cells of the progressive filteringswitching by the first elementary cells at the level of the firstfiltering blocks to control progressive switching between filtering bythe first ARMA filter and filtering by the second ARMA filter.
 14. Thedevice as claimed in claim 9, wherein within said first filteringblocks, said other elementary filtering cell is placed in parallel withsaid first elementary filtering cell and comprises a second elementaryfiltering cell originating from factorisation of a second ARMA filter.15. A non-transitory memory device comprising a computer program productstored thereon and readable by a microprocessor of a filtering device,the computer program product comprising instructions that configure themicroprocessor to perform a process for filtering a digital signal whenthis program is loaded and executed by the microprocessor, wherein theprocess comprises: receiving a digital signal by a filtering devicecomprising a plurality of first cascaded filtering blocks, eachfiltering block comprising a first elementary filtering cell and anotherassociated elementary filtering cell, all of said first elementaryfiltering cells of the plurality of first filtering blocks beingconfigured to factorise a first autoregressive moving average (ARMA)filter; filtering the digital signal by the filtering device to producea filtered digital signal, wherein filtering comprises: themicroprocessor receiving a first request for a filtering change to orfrom filtering by first ARMA filter by the filtering device, at leastone control element progressively switching the filtering device betweendigital signal filtering by the first elementary filtering cell anddigital signal filtering by said other associated elementary filteringcell at a level of each of a plurality of first cascaded filteringblocks, in response to receiving the request.